1. Field of the Invention
This invention relates in general to surface treatment of metals, particularly to those for steel requiring exceptional wear resistance under heavy loads such as those imposed upon earth boring drill bit bearings.
2. Description of the Prior Art
In my copending application, Ser. No. 387,566, I describe a method of providing a strong and tough wear resistant surface for steel articles. This method involves carburizing a machined part, boronizing over the carburized case, then hardening from a protective atmosphere and tempering. This method is particularly suitable for the bearing surfaces of earth boring bits, which are under very heavy loads.
It is known that the initial seizure point of a bearing depends to a large extent on the smoothness of the bearing surface. In view of the large loads applied to the bearings of earth boring bits, a relatively smooth surface finish is required.
Because of the extreme hardness of a boronized case, it is difficult to obtain a smooth surface. One prior art method is to use a lapping technique with diamond paste or abrasive. This is expensive and difficult. Polishing the "green" stock or metal before carburizing does not solve the problem because the carburizing and boronizing processes roughen the surface. In the prior art, polishing green stock prior to carburizing to the smoothest finish practical results in a surface finish of about 45-80 microinch RMS (root mean square) after boronizing.